I know it's old but maybe it can help for others.
You can use dtostrf function.
/***************************************************
This is a library example for the MLX90614 Temp Sensor
Designed specifically to work with the MLX90614 sensors in the
adafruit shop
----> https://www.adafruit.com/products/1748
----> https://www.adafruit.com/products/1749
These sensors use I2C to communicate, 2 pins are required to
interface
Adafruit invests time and resources providing this open source code,
please support Adafruit and open-source hardware by purchasing
products from Adafruit!
Written by Limor Fried/Ladyada for Adafruit Industries.
BSD license, all text above must be included in any redistribution
****************************************************/
#include <Wire.h>
#include <Adafruit_MLX90614.h>
Adafruit_MLX90614 mlx = Adafruit_MLX90614();
void setup() {
Particle.publish("Status", "Adafruit MLX90614 test");
mlx.begin();
char ambient[64];
char tempC[64];
dtostrf(mlx.readAmbientTempC(), 2, 2, ambient);
dtostrf(mlx.readObjectTempC(), 2, 2, tempC);
char buff[64];
sprintf(buff, "Ambient: %s Temp: %s", ambient, tempC);
dtostrf(temp, 2, 2, msg);
Particle.publish("Ambiant", buff);
}
void loop() {
}